1. Field of the Invention
The invention relates to memory display devices of the maxtrix addressing type using ferroelectric smectic liquid crystals in which a memory effect is obtained by means of a stabilizing electric field.
2. Description of the Prior Art
Liquid crystals occupy at the present time a very important place in the field of display devices. This is due in particular to the electro-optic effects provided by liquid crystals of the twisted nematic type. The great success of this effect resides in its simplicity in use, its low control voltages and its low power consumption. The main disadvantages of these devices is that, despite much research work in this field, its multiplexing rate seems limited to about 100. This is an unquestionable handicap in the use of this effect for high definition matrix screens, comprising for example several tens of lines of characters. Therefore research work has turned towards memory electro-optical effects for which the multiplexing rate is theoretically infinite.
Multiplexing rates have been substantially increased by using solutions which may be classed into two families. A first family groups together the devices comprising an integrated control matrix in which each picture element is placed in series with a non linear element of the varistor, thin layer transistor, series and opposition mounted diode type, etc . . . a second family of solutions groups together the devices which use materials having a memory electro-optical effect. Among these devices of this second family, those may be mentioned using the combined thermal and electric effect in smectic liquid crystals. Those may also be mentioned which use the effects of the electric field on a hybrid texture in a nematic crystal and whih have formed the subject matter of a patent application filed by the applicant bearing the national registration number 83 08135 filed on the May 17, 1983.
The electro-optical effects observed in ferroelectric smectic liquid crystals offers an interesting solution to the problems of intrinsic memory effects. Studies carried out in numerous laboratories have shown that an electro-optical effect could be obtained by reversing the ferroelectric field polarization in ferroelectric smectic phases. With respect to smectics A in which the molecules are normal to the planes of the smectic layers, the molecules of ferroelectric smectic liquid crystals have in general a slant with respect to the plane of the layers. A rotation of the slant direction of these molecules also occurs about the normal to the plane of the layers in a low pitch twist (a few micrometers).
CLARK AND LAGERWALL have shown, in the European patent application published under number 0 032 362 and filed on the Jan. 10, 1980, that with cells of small thickness, it is possible to induce two stable directions of orientation of the liquid crystal molecules which may form opposed optical axis and polarization fields. Switching between these two directions is achieved by applying an appropriate electric field thereto. These cells, used as elementary optical valves, have as main characteristics: that of being bistable systems so having a memory, that of possessing short response times, that of requiring low control voltages and that of having low energy consumption.
However, this type of device presents two major drawbacks. First of all it requires surface treatment of the two internal faces of the plates supporting the cell which must lead to an equivalent energy for the two textures (corresponding to the two stable directions of orientation of the molecules) so that they may exist simultaneously in the absence of an applied electric field. This treatment must however allow surface sliding of the molecules during switching from one texture to another. A surface treatment having these two properties is very difficult to achieve in a reproducible way. Secondly, these devices require blocking by limit surfaces of the twist due to the chirality of the ferroelectric liquid crystal molecules and consequently impose very small thicknesses on the liquid crystal layer. These thicknesses (less than three micrometers) are very difficult to obtain industrially over large surfaces.